1. Field of the Invention
This invention relates to a magnetic detection device which converts a magnetic state to an electric signal utilizing a magentoresistance effective element (MRE), whose electric resistance is determined by the magnetic state, for speed detection by magnetic changes etc. and to a physical quantity detection device utilizing the same.
Especially, this invention relates to a magnetic detection device for detecting the amount of magnetic change with a high accuracy and sensitivity utilizing a means which converts the change of the magnetism to a digital pulse signal.
2. Description of the Related Art
Generally speaking, a position sensor or a rotational speed sensor need to have a high level of detection accruacy, a wide range of ambient temperature of use, and simple construction. Use is now made of sensors working by detecting magnetism. The reason is that an MRE has a high sensitivity to magnetism, is relatively stable against temperature changes, and can be easily produced. In addition, in an electronic circuit for converting a magnetic state into an electric signal, the magnetic state directly determines the resistance, so it is important that the design work of the detecting on circuit also be easy. These characteristics make it effective to mount both the MRE and detection circuit on one chip.
In a conventional sensor or circuit, use is made of a resistance bridge circuit, widely used for detecting a resistance value, and a voltage amplifying circuit, which can detect and amplify an output voltage.
The output voltage of the resistance bridge circuit is stable against temperature changes. The voltage amplifying circuit is an analog circuit, however, so when the temperature changes, the characteristics of the transistor in the voltage amplifying circuit changes and the resistance of the resistor shifts, whereby an offset voltage is generated. Therefore, the range of ambient temperature in which the voltage amplifying circuit can correctly work is relatively narrow.
This problem can be alleviated to a certain extent by utilizing a temperature compensating circuit, but the construction of the circuit becomes complicated and the production efficiency is reduced. Also, when the temperature increases to a much higher level, the temperature compensating circuit itself cannot work correctly any more. Therefore, the circuit is not suitable for a circuit used, for example, for detection of rotational speed in automobiles, when the range of ambient temperature of use is wide.
Note that when this kind of magnetic detection device is used as a sensor for detecting the rotational speed of a wheel of an automobile, especially used for an antilock brake system (ABS), a wide range of temperatures under which the device can be guaranteed to normally operate is required, because the sensor is usually mounted near a brake, which is usually heated to a high temperature.
In the future, sensors will have to be built on one chip and the range of ambient temperature of use of a detection circuit will become wider, so a detection circuit which can operate stably in a wide range of ambient temperatures of use will become necessary.
Note also that the mounting position of the magnetic sensor greatly affects the output signal and the adjustment thereof is very difficult.